linux-kernel - Re: [PATCH v4 2/5] arm: add new asm macro update

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Date:	Wed, 29 Jan 2014 18:28:05 +0000
From:	Leif Lindholm <leif.lindholm@...aro.org>
To:	Will Deacon <will.deacon@....com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"grant.likely@...retlab.ca" <grant.likely@...retlab.ca>,
	"linux-efi@...r.kernel.org" <linux-efi@...r.kernel.org>,
	"linux@....linux.org.uk" <linux@....linux.org.uk>,
	"patches@...aro.org" <patches@...aro.org>,
	"roy.franz@...aro.org" <roy.franz@...aro.org>,
	"matt.fleming@...el.com" <matt.fleming@...el.com>,
	"msalter@...hat.com" <msalter@...hat.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v4 2/5] arm: add new asm macro update_sctlr

On Wed, Jan 22, 2014 at 11:20:55AM +0000, Will Deacon wrote:
> > +#ifdef CONFIG_CPU_CP15
> > +/* Macro for setting/clearing bits in sctlr */
> > +	.macro update_sctlr, set:req, clear:req, tmp:req, tmp2:req
> > +	mrc	p15, 0, \tmp, c1, c0, 0
> > +	ldr	\tmp2, =\set
> > +	orr	\tmp, \tmp, \tmp2
> > +	ldr	\tmp2, =\clear
> > +	mvn	\tmp2, \tmp2
> > +	and	\tmp, \tmp, \tmp2
> > +	mcr	p15, 0, \tmp, c1, c0, 0
> 
> I think this would be cleaner if you force the caller to put set and clear
> into registers beforehand, rather than have to do the literal load every
> time. Also, I don't think set and clear should be required (and then you can
> lose tmp2 as well).

I can't figure out how to make register-parameters non-required
(i.e. conditionalise on whether an optional parameter was provided),
so my attempt of refactoring actually ends up using an additional
register:

#ifdef CONFIG_CPU_CP15
/* Macro for setting/clearing bits in sctlr */
	.macro	update_sctlr, set:req, clear:req, tmp:req
	mrc	p15, 0, \tmp, c1, c0, 0
	orr	\tmp, \set
	mvn	\clear, \clear
	and	\tmp, \tmp, \clear
	mcr	p15, 0, \tmp, c1, c0, 0
	.endm
#endif

If you think that's an improvement I can do that, and I have (just)
enough registers to spare.
If I'm being daft with my macro issues, do point it out.

/
    Leif
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